(1) Field of the Invention
The present invention is directed to providing a warning of the existence of an unsafe condition in an area under surveillance and particularly to the detection of the presence of smoke before the smoke's density reaches a dangerous level. More specifically, this invention is directed to a condition responsive device for providing an alarm in response to smoke density in excess of a predetermined level. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for use in or as a fire alarm system. Accordingly, the invention will be described below in the context of such a system and particularly with respect to an alarm which is responsive to smoke density.
As is well known, most fatalities suffered in fires result from smoke inhalation. As is also well known, in most fatal home fires the smoke density reaches a level which is sufficiently high to incapacitate the victims before there has been adequate temperature rise to cause activation of a conventional temperature sensitive alarm system located a significant distance from the fire.
Present commercially available smoke detectors suitable for residential use may be divided into two general types based upon their principle of operation. The first general type includes those devices which utilize an ionization principle while the second type includes those devices which operate on a photoelectric refraction or light scatter principle. Ionization type smoke detectors are overly susceptible to false alarm and are insensitive to deadly cold smoke. With regard to false alarm, ionization type detectors may be false triggered by hair or deodorant spray, atmospheric changes or harmless effluent from cooking. The susceptibility to false alarm and the insensitivity to cold smoke, plus severe location restrictions, characteristic of ionization type smoke detectors have slowed the trend toward needed fire safety legislation and public acceptance. In the case of photoelectric light scatter devices, because of their rather restricted air inlets, response to the presence of smoke is relatively slow. More importantly, the sensitivity of devices which operate on a refraction principle varies with the aging of the light source, which is typically a short-lived incandescent lamp, and with different colors of smoke. In practice, the sensitivity of available smoke detectors operating on a photoelectric refraction principle will vary with the individual light source employed and it may be noted that the light emitted by lamps of the same type will vary within a predetermined tolerance range. As an additional disadvantage, photoelectric light scatter devices are, because of a characteristic high power consumption, limited to AC operation.
To be safe and acceptable, a smoke detector must not be sensitive to supply voltage, temperature, component aging or dirty optics since any of these factors could cause a false alarm or, more importantly, an increase in the smoke density at which the device will alarm. Similarly, a smoke detector must be insensitive to atmospheric shock. Further, a usable smoke detector should preferably include means to advise the user of a component failure, particularly a light source failure in the case of an optical device. Optical type devices should also be immune to false triggering by external light. All prior art smoke detectors have failed to meet one, and usually several, of these criteria.
A third known technique for the detection of the presence of smoke may generally be described as light obscuration. It has long been known that the most straightforward and accurate manner of detecting the presence of smoke is to measure the obscuration of a detector resulting from the presence of smoke between the detector and a light source. Large instruments operating on the light obscuration principle are employed in testing laboratories as the standard against which ionization and light scatter devices are rated. However, partly as a consequence of the large size and comparatively high cost of previously available obscuration type smoke detectors, devices operating on an obscuration principle have not been available for the general residential market.